<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>
    <form onsubmit="return true" oninput="o.value = a.valueAsNumber + b.valueAsNumber">
        <input name="a" type=number step=any> +
        <input name="b" type=number step=any> =
		<output id="o" for="a b"></output>
    </form>
</body>

</html>